1. Field of the Invention
The present invention relates to a catalyst consisting of a metal of the VIII group supported on acid activated carbon functionalized with sulfonic groups, a process for the direct synthesis of hydrogen peroxide from hydrogen and oxygen which uses said catalyst and the use of the hydrogen peroxide solution in oxidation processes catalyzed by titanium-silicalite.
2. Description of the Background
Hydrogen peroxide is a commercially important product which is widely used as bleaching agent in the textile and paper industries, in the environmental field as a biocide and in the chemical industry.
In particular the use of hydrogen peroxide is known in the art in a series of oxidation reactions catalyzed by titanium silicalite, such as for example the epoxidation of olefins (EP-100,119, the ammoxymation of carbonyl compounds (U.S. Pat. No. 4,794,198), the oxidation of ammonia to hydroxylamine (U.S. Pat. No. 5,320,819) and the hydroxylation of aromatic hydrocarbons (U.S. Pat. No. 4,369,783)
Generally solutions of hydrogen peroxide are used, obtained by synthesis processes which are based on the alternating oxidation and reduction of alkylanthraquinones.
These processes however, have substantial disadvantages deriving from the necessity of operating with large volumes of reagents, the numerous steps required, the relatively high cost of the intermediates and the production of inactive by-products.
As a result, other processes for the synthesis of hydrogen peroxide have been proposed in the art, among which those using a catalytic system consisting of a noble metal, preferably palladium, supported on a carrier, for the direct synthesis of hydrogen peroxide from hydrogen and oxygen.
For example, U.S. Pat. No. 4,772,458 describes a process for the synthesis of hydrogen peroxide which uses a catalyst based on palladium supported on coal, in which the reaction is carried out in water in the presence of hydrogen ions and bromide ions in a molar ratio of at least 2:1.
The H.sup.+ and Br.sup.- ions are supplied as a combination of a strong acid such as sulfuric, phosphoric, hydrochloric or nitric acid, and a bromine salt.
Quantities of acid ranging from 2.5 to 10 g/litre are generally required to obtain high concentrations of H.sub.2 O.sub.2.
The use of high quantities of acid, in addition to creating problems of a technological nature, for example corrosion of the autoclaves, also causes serious problems relating to the dissolution of the active phase (metal) of the catalyst in the reaction medium, with a consequent reduction in the activity and life of the catalyst itself.
The metal dissolved in the reaction medium, moreover, is difficult to recover with the recycling methods at present available.
In addition, the process operates under critical conditions as concentrations of H.sub.2 &gt;5% (generally 18%) with respect to the reaction mixture, are used, which is therefore within the explosivity limits of H.sub.2 /O.sub.2 mixtures.
U.S. Pat. No. 4,889,705 relates to an improved process for the synthesis of hydrogen peroxide in which the H.sup.+ and Br.sup.- ions are supplied directly as HBr. Operating according to this process Quantities of Br.sup.- ions of more than 10.sup.-4 moles/litre are required to obtain hydrogen peroxide. The use of high quantities of halides creates problems of instability of both the catalyst metal and the hydrogen peroxide solution produced.
To overcome these drawbacks alternative processes have been proposed which can be carried out without acids and/or bromine ions in the reaction medium.
For example, patent application EP-492,064 describes a process for the synthesis of hydrogen peroxide from hydrogen and oxygen, which uses a catalyst based on palladium supported on a halogenated resin, in particular a brominated styrene/divinylbenzene resin.
The reaction is carried out in water, in the presence of an acid selected from sulfuric, phosphoric or nitric acid. Operating with this process however, concentrations of H.sub.2 O.sub.2 of about 0.58% are obtained.
EP-504741 describes a process for the synthesis of H.sub.2 O.sub.2 front hydrogen and oxygen which uses a catalyst based on palladium or platinum supported on an acid or superacid carrier selected from molibden, zirconium or tungsten oxides.
The reaction is carried out in water, in the presence of a promoter selected from bromine salts in quantities of more than 0.1 mmoles/1 of reaction mixture. Operating according to this process hydrogen peroxide is obtained in concentrations not higher than 1%.
U.S. Pat. No. 5,320,921 describes a process for the synthesis of H.sub.2 O.sub.2 from hydrogen and oxygen which uses a catalyst based on palladium or platinum supported on a heteropolyacid made insoluble in water. The reaction is carried out in water in the presence of bromine ions (0.5 mmoles/litre of sodium bromide). Quantities of H.sub.2 O.sub.2 equal to about 1.1% are obtained.
These processes of the known art, therefore, do not have a productivity, referring to the quantity of hydrogen peroxide produced, which is sufficiently high for their application on an industrial scale to be of interest.